Proximity Detection Using an Antenna and Directional Coupler Switch

ABSTRACT

Detection of an increase in a mismatch of an antenna of a radio frequency (RF) device and/or a change in a capacitance value of the antenna indicates proximity of a body to the antenna. Upon detection of proximity of a body to the antenna, reduction of transmit power of the RF device may be done to meet Specific Absorption Rate (SAR) level regulations.

RELATED PATENT APPLICATION

This application claims priority to commonly owned U.S. ProvisionalPatent Application Ser. No. 61/598,969; filed Feb. 15, 2012; which ishereby incorporated by reference herein for all purposes.

TECHNICAL FIELD

The present disclosure relates to a radio frequency (RF) device having adirectional coupler switch between the RF device and an antenna, and, inparticular, utilizing the directional coupler switch and antenna forproximity detection of a user.

BACKGROUND

Wi-Fi is a flexible, short-range data communications technology used toconnect devices as diverse as notebooks, tablets, handsets, consumerelectronics, smart utility meters and much more. Wi-Fi technology iswidely used to provide wireless internet access in public places likeairports, hotels and shopping centers, and is also used in the home andoffice to allow a wide range of devices to access the internet andnetwork with each other without the need for special cables. Wi-Fidevices use low-power radio waves in the 2.4 and 5 GHz range to transmitand receive data over the air. Wherein the greater the Wi-Fi transmitpower, the long the range of the Wi-Fi enabled device. However, Wi-Fidevices operate at frequencies that may be potentially harmful to humanswhen enough radio frequency (RF) power output is produced from the Wi-Fidevice. The FCC and other federal governmental agencies around the worldrequire that any wireless device be evaluated to meet RF exposure limitsset forth in governmental regulations, e.g., Specific Absorption Rate(SAR) levels.

Therefore a SAR test is necessary in determining maximum allowable RFpower output when the Wi-Fi enabled device is in close proximity to auser. The Specific Absorption Rate (SAR) is the unit of measurement forthe amount of radio frequency (RF) absorbed by the body when using awireless device. The SAR value is expressed in terms of watts perkilogram (W/kg) or milliwatts per gram (mW/g). The RF exposure limitsused are expressed in the terms of SAR, which is a measure of theelectric and magnetic field strengths and power density for transmittersoperating at frequencies from 300 kHz to 100 GHz. The most generallyaccepted method for measuring SAR values is the direct method SAR test.This method utilizes a model called a “SAM phantom” to simulate thehuman head and a “flat phantom” to simulate the human body. With thismethod, wireless devices are tested at the highest certified power levelin laboratory conditions utilizing a SAR test system with a robot.

There is a potential risk that RF-emissions may cause long term healthissues. For this reason it is important to detect the presence of anabsorbing body. Several recent medical studies point to the potentialcancer causing effects of absorbed RF radiation from the antenna ofcommon portable devices such as Cell Phones, E-Readers and TabletComputers. Therefore, with Wi-Fi enabled devices, it may not be safe fora user of the device if the Wi-Fi feature of the device is in operationat its maximum RF power output when in close proximity to the user'sbody. New FCC test guidelines require the measurement of RF radiation ata distance of about 10 millimeters from each device surface duringoperation.

SUMMARY

Hence, there exists a need for proximity detection systems which canreliability detect a human user within these FCC distances and reducethe transmit power to meet these stricter FCC guidelines. Furthermore,there exists a need in RF devices to provide for the ability to allow RFforward and reflected power measurements of the respective device in asimple and efficient manner.

According to an embodiment, a directional coupler for coupling anantenna with a radio frequency (RF) system may comprise: a firstconnection for coupling the antenna to a directional coupler switcharranged within the directional coupler; a second connection forcoupling the RF system to the antenna through the directional couplerswitch; and a third connection for coupling the antenna to a capacitancemeasurement device through the directional coupler switch, wherein thecapacitance measurement device measures capacitance values of theantenna.

According to a further embodiment, a return loss bridge may be coupledbetween the directional coupler switch and the first connection.According to a further embodiment, the return loss bridge may provide areflected power measurement. According to a further embodiment, avoltage standing wave ratio bridge may be coupled between thedirectional coupler switch and the first connection. According to afurther embodiment, the voltage standing wave ratio bridge may provide areflected voltage standing wave measurement. According to a furtherembodiment, the directional coupler switch may be a three positionswitch.

According to a further embodiment, the capacitive measurement device maycomprise a charge time measurement unit (CTMU). According to a furtherembodiment, the capacitive measurement device may comprise a capacitivevoltage divider (CVD) circuit. According to a further embodiment, thecapacitive measurement device may comprise an oscillator and frequencydiscriminator circuit. According to a further embodiment, amicrocontroller may be coupled to and control the directional couplerswitch.

According to another embodiment, a radio frequency device may comprise:a radio frequency subsystem; a power amplifier coupled to the radiofrequency subsystem; a low noise amplifier coupled to the radiofrequency subsystem; an antenna; a directional coupler may comprise: afirst connection for coupling the antenna to a directional couplerswitch arranged within the directional coupler; a second connection forcoupling the power amplifier to the antenna through the directionalcoupler switch; a third connection for coupling the low noise amplifierto the antenna through the directional coupler switch; and a fourthconnection for coupling the antenna to a capacitance measurement devicethrough the directional coupler switch, wherein the capacitancemeasurement device measures capacitance values of the antenna.

According to a further embodiment, a return loss bridge may be coupledbetween the directional coupler switch and the first connection.According to a further embodiment, the return loss bridge may provide areflected power measurement. According to a further embodiment, avoltage standing wave ratio bridge may be coupled between thedirectional coupler switch and the first connection. According to afurther embodiment, the voltage standing wave ratio bridge may provide areflected voltage standing wave measurement. According to a furtherembodiment, the directional coupler switch may be a three positionswitch.

According to yet another embodiment, a method for determining proximityof a object to an antenna of a radio frequency device may comprise thesteps of: measuring a capacitance value of the antenna; determiningwhether the measured capacitance value of the antenna has changed from aprevious measurement of a capacitance value of the antenna; anddetecting proximity of the object to the antenna when the capacitivevalue of the antenna has changed by at least a certain value.

According to a further embodiment of the method, may comprise the stepsof: measuring a reflected voltage standing wave value of the antenna;determining whether the reflected voltage standing wave value of theantenna has changed from a previous measurement of a reflected voltagestanding wave value of the antenna; and detecting proximity of theobject to the antenna when the reflected voltage standing wave value ofthe antenna has changed by at least a certain value.

According to a further embodiment of the method, may comprise the stepof reducing transmit power of the radio frequency device when detectingproximity of the object to the antenna.

According to still another embodiment, a method for determiningproximity of a object to an antenna of a radio frequency device maycomprise the steps of: measuring a reflected voltage standing wave valueof the antenna; determining whether the reflected voltage standing wavevalue of the antenna has changed from a previous measurement of areflected voltage standing wave value of the antenna; and detectingproximity of the object to the antenna when the reflected voltagestanding wave value of the antenna has changed by at least a certainvalue.

According to a further embodiment of the method, may comprise the stepsof: measuring a capacitance value of the antenna; determining whetherthe measured capacitance value of the antenna has changed from aprevious measurement of a capacitance value of the antenna; anddetecting proximity of the object to the antenna when the capacitivevalue of the antenna has changed by at least a certain value. Accordingto a further embodiment of the method, may comprise the step of reducingtransmit power of the radio frequency device when detecting proximity ofthe object to the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure may be acquiredby referring to the following description taken in conjunction with theaccompanying drawings wherein:

FIG. 1 illustrates a schematic block diagram of a RF device, accordingto a specific example embodiment of this disclosure;

FIG. 2 illustrates a more detailed schematic block diagram of a portionof the RF device shown in FIG. 1, according to a specific exampleembodiment of this disclosure; and

FIG. 3 illustrates more detailed alternate schematic block diagrams of aportion of the RF device shown in FIG. 1, according to the teachings ofthis disclosure.

While the present disclosure is susceptible to various modifications andalternative forms, specific example embodiments thereof have been shownin the drawings and are herein described in detail. It should beunderstood, however, that the description herein of specific exampleembodiments is not intended to limit the disclosure to the particularforms disclosed herein, but on the contrary, this disclosure is to coverall modifications and equivalents as defined by the appended claims.

DETAILED DESCRIPTION

According to various embodiments, a mismatch condition of an antenna maybe used to detect a body in the near field of the transmitting antenna.This allows either to reduce the transmit power or shutting thetransmitting down while the device is in a severe enough mismatchcondition. According to various further embodiments, capacitiveproximity or touch sensing using the antenna as a capacitive sensor maybe used to detect an object (body), e.g., RF device user, upon a greatenough capacitive change of the capacitive sensor antenna. Similarly, aproximity detection may be used to reduce transmit power to meet SARrequirements.

According to various embodiments, an integrated feature can be providedto RF products that would allow for proximity or antenna performance tobe measured without additional systems/devices. A majority of capacitiveproximity solutions may be used to detect the proximity of a human bodyto an antenna in order to reduce the output power of the RF amplifier topass FCC SAR regulations. Since the antenna is the element of interestfor proximity in the RF system, this would allow the antenna to becomethe proximity sensor. Utilizing the antenna as a capacitive sensorreduces system cost, improves the capacitive sensing performance andenables higher packaging density by eliminating the need for a separatecapacitance sensor. The reflected power output analog signal and/or theantenna used as a capacitive sensor may be post processed in a digitalprocessor/microcontroller function.

According to an embodiment, an additional port on an RF directionalcoupler switch may be used to couple the antenna to a capacitancemeasurement circuit for detecting proximity of a body to the antenna.This additional port may be used as a capacitance sense path for theantenna to used as a capacitive touch or capacitive proximity sensor.

The various embodiments proposed may be used in any RF product.According to an embodiment, an additional input-output port on thedirectional coupler switch could be provided as an external pin on thedevice to allow for reflected power of the antenna to be measured or asa path to use the antenna as a capacitive sensor by an external device.This may provide an integrated feature RF products that is unique anduseful in the market. It also may provide a simple, low cost solution tomeeting FCC SAR regulations.

Various embodiments disclosed herein may be based on a directionalcoupler between the power amplifier and the antenna. Such directionalcouplers are already present in some conventional chipsets, which usesthem for tuning the antenna to best performance. According to variousembodiments, this mismatch information may also be used to detect afield disturbing presence in the near field of the antenna to evaluatethe presence or proximity of a human body. Capacitive proximity sensingwith the antenna as a capacitive sensor may be used without requiringtransmit power to be activated so that RF power output may be adjustedfor an appropriate SAR level before a transmission occurs.

Referring now to the drawings, the details of specific exampleembodiments are schematically illustrated. Like elements in the drawingswill be represented by like numbers, and similar elements will berepresented by like numbers with a different lower case letter suffix.

Referring to FIG. 1, depicted is a schematic block diagram of a RFdevice, according to a specific example embodiment of this disclosure. ARF device 100 may comprise a directional coupler 102, a power amplifier(PA) 118 (low pass filter and matching circuits not shown), a RFsubsystem 120, a low noise amplifier (LNA) 116, a capacitancemeasurement device 114, a digital processor and memory 112 and a RFantenna 104. The directional coupler 102 may comprise a directionalcoupler switch 110, a return loss bridge 106 or voltage standing waveratio (VSWR) bridge 106 a, and a VSWR bridge interface 108 coupled tothe digital processor 112. The RF subsystem 120 may be a digitalwireless system, e.g., Wi-Fi, etc.

Referring to FIG. 2, depicted is a more detailed schematic block diagramof a portion of the RF device shown in FIG. 1, according to a specificexample embodiment of this disclosure. The directional coupler switch110 may comprise a multi-throw switch 222 that couples the antenna 104through the VSWR bridge 106 a to either the PA 118 or the LNA 116 fortransmit or receive operation, respectively, of the RF device 100. Anadditional port may be added to the RF directional coupler switch 110and coupled to the antenna 104 through the switch 222. This additionalport may function as a capacitance sense path so that the antenna may beused as a capacitive touch or proximity sensor. When the switch 222 isin the position shown in FIG. 2, the antenna 104, acting as a capacitivesensor, is coupled to the capacitance measurement device 114. Thecapacitance measurement device 114 measures the capacitance of thecapacitive sensor antenna 110, and when a sufficient change in thecapacitance value of the antenna 110, acting as a capacitive sensor,occurs a signal may be sent to the digital processor 112 indicating thatan object is in close proximity to or touching the antenna 110. Thedigital processor 112 may then use this capacitance change informationfrom the capacitance measurement circuit 112 to adjust down the transmitRF power to be in compliance with the SAR regulations. The switch 222may be controlled by the digital processor 112.

The capacitance measurement device 114 may be any one or morecapacitance measurement devices that have the necessary capacitanceresolution for this application. For example, but not limited to, aCharge Time Measurement Unit (CTMU) may be used for very accuratecapacitance measurements. The CTMU is more fully described in Microchipapplications notes AN1250 and AN1375, available at www.microchip.com,and commonly owned U.S. Pat. No. 7,460,441 B2, entitled “Measuring along time period;” and U.S. Pat. No. 7,764,213 B2, entitled“Current-time digital-to-analog converter,” both by James E. Bartling;wherein all of which are hereby incorporated by reference herein for allpurposes.

Also the capacitance measurement device 114 may be used to just detect achange in the capacitance of the antenna 104. For example, a CapacitiveVoltage Divider (CVD) device may be used according to AN1298, availableat www.microchip.com, and commonly owned U.S. Patent ApplicationPublication No.: US 2010/0181180 A1, entitled “Capacitive Touch SensingUsing an Internal Capacitor of an Analog-to-Digital Converter (ADC) anda Voltage Reference” by Dieter Peter. A Capacitive Sensing Module (CSM)circuit may be used according to AN1171, AN1312 and AN1334, available atwww.microchip.com, and commonly owned U.S. Patent Application No.: US2011/0007028 A1, entitled “Capacitive Touch System With Noise Immunity”by Keith E. Curtis, et al.; wherein all of which are hereby incorporatedby reference herein for all purposes.

Another capacitive change detection circuit may be a tuned circuit usingthe capacitance of the antenna 104 as one of the frequency determiningelements and a frequency discriminator circuit, as more fully describedin commonly owned U.S. Patent Application Publication No.: US2008/0272826 A1, entitled “Interrupt/Wake-Up of an Electronic Device ina Low Power Sleep Mode When Detecting a Sensor or Frequency SourceActivated Frequency Change” by Zacharias Marthinus Smit, et al., and ishereby incorporated by reference herein for all purposes.

It is contemplated and within the scope of this disclosure that onehaving ordinary skill in the art of capacitive measurement andcapacitive change detection circuits and having the benefit of thisdisclosure could design an effective capacitive measurement and/orcapacitive change detection circuit and apply it according to theteachings of this disclosure. It is also contemplated and within thescope of this disclosure that the VSWR interface 108, digital processorand memory 112 and capacitance measurement device 114 may be provided ina microcontroller.

Referring to FIG. 3, depicted are more detailed alternate schematicblock diagrams of a portion of the RF device shown in FIG. 1, accordingto the teachings of this disclosure. Operation of a return loss bridge106 and a VSWR bridge 106 a are interchangeable in this application.Referring to FIG. 3( a), the VSWR interface 108 a may comprise amultiplexer 332 and an analog-to-digital converter (ADC) 330. Themultiplexer 330 may be controlled by the digital processor 112. Themultiplexer 330 may have two inputs, one coupled to the forward standingwave voltage and the other coupled to the reverse standing wave voltagefrom the VSWR bridge 106 a. The digital processor 112 selects throughthe multiplexer 332 which standing wave voltage the ADC 330 will convertto a digital representation thereof and then reads that digitalrepresentation. From the standing wave voltages, the digital processorcan determine proximity of a body (not shown) to the antenna 104, and/orcontrol the directional coupler switch so that the antenna 104 iscoupled to the capacitance measurement device 114 to determine whetherthe antenna capacitance has changed sufficiently to indicate proximityof a body thereto.

Referring to FIG. 3( b), the VSWR interface 108 b may comprise a voltagecomparator 340 and voltage divider resistors 342 and 344. Normally theforward VSWR will be at a much higher voltage than the reverse VSWRvoltage from the VSWR bridge 106 a, however when a mismatch occurs atthe antenna 104 the reverse VSWR voltage will increase. Selection of theresistance values of the voltage divider resistors 342 and 344 may bedetermined by a desired “trip” reverse VSWR voltage. When this tripvoltage is exceeded the voltage comparator 340 sends a logic high “1” tothe digital processor 112. When a high reverse VSWR voltage is detected,the digital processor may determine proximity of a body (not shown) tothe antenna 104, and/or control the directional coupler switch so thatthe antenna 104 is coupled to the capacitance measurement device 114 todetermine whether the antenna capacitance has changed sufficiently toindicate proximity of a body thereto.

While embodiments of this disclosure have been depicted, described, andare defined by reference to example embodiments of the disclosure, suchreferences do not imply a limitation on the disclosure, and no suchlimitation is to be inferred. The subject matter disclosed is capable ofconsiderable modification, alteration, and equivalents in form andfunction, as will occur to those ordinarily skilled in the pertinent artand having the benefit of this disclosure. The depicted and describedembodiments of this disclosure are examples only, and are not exhaustiveof the scope of the disclosure.

What is claimed is:
 1. A directional coupler for coupling an antennawith a radio frequency (RF) system, comprising: a first connection forcoupling the antenna to a directional coupler switch arranged within thedirectional coupler; a second connection for coupling the RF system tothe antenna through the directional coupler switch; and a thirdconnection for coupling the antenna to a capacitance measurement devicethrough the directional coupler switch, wherein the capacitancemeasurement device measures capacitance values of the antenna.
 2. Thedirectional coupler according to claim 1, further comprising a returnloss bridge coupled between the directional coupler switch and the firstconnection.
 3. The directional coupler according to claim 2, wherein thereturn loss bridge provides a reflected power measurement.
 4. Thedirectional coupler according to claim 1, further comprising a voltagestanding wave ratio bridge coupled between the directional couplerswitch and the first connection.
 5. The directional coupler according toclaim 4, wherein the voltage standing wave ratio bridge provides areflected voltage standing wave measurement.
 6. The directional coupleraccording to claim 1, wherein the directional coupler switch is a threeposition switch.
 7. The directional coupler according to claim 1,wherein the capacitive measurement device comprises a charge timemeasurement unit (CTMU).
 8. The directional coupler according to claim1, wherein the capacitive measurement device comprises a capacitivevoltage divider (CVD) circuit.
 9. The directional coupler according toclaim 1, wherein the capacitive measurement device comprises anoscillator and frequency discriminator circuit.
 10. The directionalcoupler according to claim 1, further comprising a microcontrollercoupled to and controlling the directional coupler switch.
 11. A radiofrequency device, comprising: a radio frequency subsystem; a poweramplifier coupled to the radio frequency subsystem; a low noiseamplifier coupled to the radio frequency subsystem; an antenna; and adirectional coupler comprising a first connection for coupling theantenna to a directional coupler switch arranged within the directionalcoupler; a second connection for coupling the power amplifier to theantenna through the directional coupler switch; a third connection forcoupling the low noise amplifier to the antenna through the directionalcoupler switch; and a fourth connection for coupling the antenna to acapacitance measurement device through the directional coupler switch,wherein the capacitance measurement device measures capacitance valuesof the antenna.
 12. The radio frequency device according to claim 11,further comprising a return loss bridge coupled between the directionalcoupler switch and the first connection.
 13. The radio frequency deviceaccording to claim 12, wherein the return loss bridge provides areflected power measurement.
 14. The radio frequency device according toclaim 11, further comprising a voltage standing wave ratio bridgecoupled between the directional coupler switch and the first connection.15. The radio frequency device according to claim 14, wherein thevoltage standing wave ratio bridge provides a reflected voltage standingwave measurement.
 16. The radio frequency device according to claim 11,wherein the directional coupler switch is a three position switch.
 17. Amethod for determining proximity of a object to an antenna of a radiofrequency device, said method comprising the steps of: measuring acapacitance value of the antenna; determining whether the measuredcapacitance value of the antenna has changed from a previous measurementof a capacitance value of the antenna; and detecting proximity of theobject to the antenna when the capacitive value of the antenna haschanged by at least a certain value.
 18. The method according to claim17, further comprising the steps of: measuring a reflected voltagestanding wave value of the antenna; determining whether the reflectedvoltage standing wave value of the antenna has changed from a previousmeasurement of a reflected voltage standing wave value of the antenna;and detecting proximity of the object to the antenna when the reflectedvoltage standing wave value of the antenna has changed by at least acertain value.
 19. The method according to claim 17, further comprisingthe step of reducing transmit power of the radio frequency device whendetecting proximity of the object to the antenna.
 20. A method fordetermining proximity of a object to an antenna of a radio frequencydevice, said method comprising the steps of: measuring a reflectedvoltage standing wave value of the antenna; determining whether thereflected voltage standing wave value of the antenna has changed from aprevious measurement of a reflected voltage standing wave value of theantenna; and detecting proximity of the object to the antenna when thereflected voltage standing wave value of the antenna has changed by atleast a certain value.
 21. The method according to claim 20, furthercomprising the steps of: measuring a capacitance value of the antenna;determining whether the measured capacitance value of the antenna haschanged from a previous measurement of a capacitance value of theantenna; and detecting proximity of the object to the antenna when thecapacitive value of the antenna has changed by at least a certain value.22. The method according to claim 20, further comprising the step ofreducing transmit power of the radio frequency device when detectingproximity of the object to the antenna.